The early bombardment of the inner solar system played a critical role in the evolution of Earth’s biosphere. To glean insights into astrobiology issues, however, one first needs to know what happened when. To date, this has been problematic — the most commonly-used and cited bombardment models (i.e., impacts from a “terminal cataclysm” ~3.9 Gyr ago; impacts from a monotonically-decreasing impactor population derived from planet formation processes) do not match constraints. Fortunately, NASA’s Lunar Science Institute and SSERVI programs have allowed many to explore these issues in depth. This has led to a new bombardment model consistent with constraints stretching from Mercury to the asteroid belt (e.g., Bottke et al. 2012; Nature).
This model’s predictions for Earth are surprising and interesting, with many events in Earth’s early history coincident with impacts in some fashion. Examples include: (i) the so-called “late heavy bombardment” (LHB) that made the youngest lunar basins started ~4.1-4.2 Gyr ago, the same time that most Hadean-era zircons formed; (2) the LHB endgame took place 1.8-2.8 Gyr ago, the same interval when major changes were afoot in Earth’s biosphere (e.g., “whiffs” of oxygen, the rise of oxygen ~2.4 Gyr ago, early snowball Earth episodes); (iii) relatively few large impacts may have taken place ~0.8-1.8 Gyr ago, the same time when Earth’s oceans may have been dominated by euxinic conditions for long periods.
While these coincidences are intriguing, causation cannot be argued without additional evidence or associated modeling work. The goal of this talk is to solicit feedback from astrobiologists on these issues, with the hope that it will lead to new ideas and collaborative work for our diverse communities.
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